Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or more unit cells, for example, three or four unit cells, for each device. To the contrary, medium- or large-sized devices, such as vehicles, use a medium- or large-sized battery system comprising a plurality of unit cells electrically connected with each other as a high-output, large-capacity battery is required.
As the unit cell for the medium- or large-sized battery system is widely used a pouch-shaped secondary cell, which is generally packaged in a battery case made of a laminate sheet consisting of aluminum and polymer resin, and therefore, the mechanical strength of the pouch-shaped secondary cell is not high. For this reason, a plurality of unit cells are mounted in a battery cartridge, where the unit cells are connected in series or parallel, and a plurality of battery cartridges are electrically connected with each other to constitute a battery module.
A battery module widely used for hybrid electric vehicles has a closed structure in which an electrolyte cannot leak from each unit cell (secondary cell). That is to say, the closed type battery module has an advantage of preventing leakage of the electrolyte. However, the closed type battery module has problems in that heat generated in unit cells is accumulated, and the accumulated heat accelerates the deterioration of the cells and that a small or large number of unit cells are overheated when they are operated abnormally, and therefore, the unit cells may fire or explode. Lithium-ion secondary cells or lithium-ion polymer secondary cells, which are currently generating much interest in the potential uses of unit cells, generate heat while the secondary cells are charged and discharged. When such heat is continuously accumulated in the unit cell, the deterioration of the unit cell is accelerated, and furthermore, the unit cell may fire or explode.
Consequently, it is necessary to provide a medium- or large-sized battery system that is capable of stably mounting unit cells having low mechanical strength, effectively removing heat from the unit cells, and preventing the unit cells from overheating when the unit cells are operated abnormally, thereby improving the service life and safety of the unit cells.